Natural laminar-to-turbulent transition inside an electrically heated circular tube
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摘要: 分析了在一个电加热圆管内的自然转捩流动和对流传热。对于圆管内的流动，提出在径向脉动速度不随流动模式变化的假设下，自然转捩流动是充分发展的层流与湍流流动按照比例的合成。采用合成比例来描述该合成流动，合成比例在转捩区间会发生振荡。根据最小熵产生准则得到自然转捩发展演化的方程，其中转捩发展演化的控制因素，是合成比例的振荡。给出了一个与测量结果一致的合成比例的振荡函数，包括圆管内转捩过程的传热实验测量和速度及其脉动统计特性的实验测量。指出圆管内层流向湍流的转捩过程，可以与热力学平衡系统的连续相变过程进行比较，并且在电加热圆管内的流体，其速度和温度可以有相似和独立的转捩演化过程。Abstract: The natural laminar-to-turbulent transitional flow and convective heat transfer inside an electrically heated circular tube are analyzed. It is proposed that the transitional flow can be decomposed into the fully developed laminar flow and the turbulent flow, under the assumption that the fluctuating velocity in the radial direction does not change with varying flow modes. The composite ratios are adopted to define the composite flow, and they fluctuate during the flow transition. The minimum entropy production criterion is used to derive an equation which can describe the evolution of the transitional flow. It is deduced that the transitional behavior is governed by the fluctuations of the composite ratios. One fluctuation function is given to attain agreements with measurements including those obtained in heat transfer and flow experiments. It is pointed out that the process of the laminar-to-turbulent transition inside the tube can be compared with continuous phase transitions in a thermodynamic equilibrium system, and similar and separate processes for the transitions of the velocity and temperature inside the tube can be allowed.
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Key words:
- transitional flow /
- fluctuations /
- composition of flows /
- minimum entropy production
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图 2 管内密度、粘性系数、热传导系数和Re的变化
Figure 2. Density, viscosity, thermal conductivity and Re inside the tube
图 3 电加热管实验中的外壁温和Nu数对比
Figure 3. Outer wall temperature and Nu comparisons in an electrically heated tube test
图 5 中心线上轴向速度脉动强度的大的过冲
Figure 5. The large overshoot of I at the center line in transition region
图 7 中心线上轴向速度统计平均值的计算与测量对比
Figure 7. The calculated and measured mean values of longitudinal velocity at the center line
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